This proposal is designed to investigate the mechanism of tumor cells escape from host immune defense mechanism - especially the effector function of the immune system, and the possibility of controlling this mechanism. Specific aims are to identify and isolate the factor(s) which will inhibit or block the antibody-dependent cellular cytotoxicity (ADCC) and to produce antibody against such blocking-factor. The experimental approaches are: (1) isolation of ADCC-blocking factor(s), (2) studies of its blocking mechanisms, (3) production of anti-blocking factor antibody, and (4) therapeutic trial on un-blocking by antibodies. Special effort will be made to isolate the Fc receptor from tumor cell culture supernatants and ascitic fluids of tumor bearing animals. It is very possible the free Fc receptor could be a novel and important blocking factor released by tumor cells and/or host lymphoblasts. It is also important to explore the phenomenon of ADCC in vivo since there has been no such model available for studying anti-tumor immunity. If the working hypothesis of this proposal is correct and the mechanisms of tumor escape from the destructive efforts of the immune system were understood, it may be possible to investigate the means of controlling the tumor growth.